1. Field of the Invention
The present invention relates generally to pinless connector structures and, more particularly, to solder connection structures.
2. Description of Related Art
The electronics manufacturing industry is moving away from the use of pins as a means of connecting electronic integrated circuit (IC) packages to substrates. Fabrication of progressingly small electronic components realizes input/output limitations. In addition, pin connectors have an unacceptably high failure rate. To overcome these and other quality limitations and physical limitations of IC packages and pin connectors, surface mountable solder connection structures were developed to couple the IC package to the substrate.
An IC package that utilizes surface mountable solder connection structures is referred to as a grid array package. Today, grid array packages are utilized in electronic equipment ranging from computers to telephone switches. Mounting the grid array package to the substrate requires multiple solder connection structures. An electronic assembly resulting from the attachment of the grid array package to the substrate via solder connection structures is typically manufactured as stated hereinbelow.
The solder connection structure generally consists of a ball-shaped solder portion ("ball") electrically connected between the faying surfaces of two substrates by solder fillets. Prior to attachment and creation of the solder connection structure, the solder fillets are in the form of a paste ("paste"). The ball typically has a higher melting point than the paste. The electronic assembly may be fabricated by, first, placing balls in contact with paste disposed on the faying surfaces of the grid array package and, second, applying heat above the melting point of the paste, but below the melting point of the balls. As a result, the molten paste wets or attaches the balls to the faying surfaces of the grid array package and later solidifies into fillets. The electronic assembly can then be completed by similarly disposing paste on the corresponding faying surfaces of a substrate, contacting the area of the balls opposing the grid array package to the paste, and similarly applying the required heat. The previous method as well as other manufacturing methods are more fully described in, for example, U.S. Pat. No. 5,060,844, "Interconnection Structure and Test Method" by Behun et al.
Solder connection structures have a limited life due to cumulative weakening caused by temperature induced shear stresses and strains. Solder connection structures often must attach substrates having different compositions, for example, grid array packages are typically comprised of a ceramic while printed circuit boards (PCBs) are mostly comprised of an epoxy. Because different substrates tend to expand at different rates with respect to changes in temperature, solder connection structures must be compliant to accommodate such expansions without mechanical failure. Because the electronic apparatuses that employ the solder connection structures will typically undergo temperature changes from cyclical operation, the solder connection structures must also be fatigue resistant.
Prior art solder connection structures are typically comprised of alloys containing two elements, tin and lead. The ball is comprised of about 90% lead and 10% tin. The paste and the resulting fillets are comprised of about 63% tin and 37% lead. The particular compositions of the ball and the paste are chosen because they facilitate wetting and attachment of surfaces during heating. Also, the ball and the paste components of prior art solder connection structures have similar stress-strain performance, causing them to behave as a single entity. This can be seen by comparing shear strength and tensile strength to elongation, which can be used to define compliancy (see Manko, "Solders and Soldering," 2d Edition, McGraw-Hill, pg. 132, Table 4-3).
Because prior art solder connection structures are comprised mainly of tin and lead alloys, they lack the necessary compliancy and fatigue resistance to withstand the temperature-related expansions and contractions of the substrates of an electronic assembly during the operation thereof. Therefore, what is needed is a cost-effective, manufacturable solder connection structure that is more compliant and fatigue resistant and able to withstand temperature induced shear stresses.